US1828848A - Production of chemical wood pulps - Google Patents

Description

Oct. '27, 1931. G. A. RICHTER 1,828,848

PRODUCTION OF CHEMICAL WOOD PULPS I Original Filed March 9. 1929 /I/a,0// mam r Patented ()ct. 2'], 1931 GEQRGJE A. RICHTER, F BERLIN, NEW HAMPSHIRE, ASSIGNOR T0 BROWN COMPANY, 0F BERLIN, NEW HAMPSHIRE, A CORPORATION 0F MAINE rnonucrrou or cnniuroar. woon runes Application filed March 9, 1929, Serial No. 345,869. Renewed March 27, 1931.

This invention relates to the production of chemical wood pulps by cooking in liquors of an alkalinetype, its object being the proat elevated temperature and under pressure in a liquor containing caustic soda and sodium sulphide in solution, both these chemicals reacting upon and dissolving the cementitious material which binds the Wood fibers together. is more drastic than sodium sulphide in its action on cellulose fiber, and accordingly that the higher the sulphidity of the cookina liquor. the better the quality of the resulting pulp. l have observed that degradation of the cellulose fiber o curs during the last stages of cooking, as at this time most of the cemeutitious material present in the original wood has been dissolved. so that the caustic soda no longer reacts with a high;

degree of selectivity upon ,such material,

but reacts appreciably upon and degrades the fiber.

- In accordance with the process of the present invention, therefore, wood or other raw cellulo ic material is first cooked in a solution of caustic soda to effe t a removal of a portion of the cementitious material, but

without effecting fiber liberation. and is then cooked in'a solution of sodium" sulphide to effect fiber liberatiomas it has been found that with this process the resulting pulp is of much betterthan usual quality: Such a process permits a removalof most of those cementitious materials which are reacted upon and dissolved readily by caustic soda solution, but without degrading the fiber, as

the caustic soda solution reacts with a high degree of sele tivity upon the cementitious material of the wood before so much of it has been removed as to resultin fiber liberation. After the initial cooking in caustic soda solution, the wood isfin jcondition to be readily converted into-"pulp and when cooked in the milder sodium sulphide solu- It is known that the caustic sodafor the recovery of their valuable constituents, the spent caustic soda solution and the ill)- spent sodium sulphide solution being mixed and their valuable inoraanic content smelted under reducing conditions to produce a smelt of sodium carbonate and sodium sulphide.

The sodium carbonate may be separated from the smelt as by selective crystallization. and then causticized for use in a subsequent first cooking step, while the rest of the smelt may be sed in a subsequent second cooking step. The raw material from wh ch the chemicals are prepared for carrying out the process of the present invention may be similar to those used in the kraift process, losses taking place in the cycle being made. up by adding sodium and sulphur constituent as in the-form of sodium sulphate to the mixed spent liquors before delivery into the smeltinq furnace. onif desired. by addinfz caustic soda to the fresh caustic soda cooking liquor and sulphur to the mixed spent'liquors. In lieu of causticizing the smelted mixture; of sodium carbonate and sodium sulphide. however, the sodium carbonate may be separated from the sodium sulphide as.

by selective crystallization, so that it may be lndependently causticized and the resulting caustic soda used in carrying out a subsequent independent initial cook 0? fresh,

Wood, whereupon the wood may then be cooked in a solution of sodium sulphide-prepared from the remainder of the smelt.

The process of'the pre ent invention may best be understood from the following moredelivered to an evaporator 4 with sufidcient caustic soda solution .to maintain the wood immersed therein. The solution may vary in its alkalinity, containing, say, from about .5 to 5 pounds of caustic soda calculated as Na i) per cubic foot, weak solutions preferably being employed when cooking is to be collected under super-atmospheric pressure, and strong solutions preferably being employed when cooking is to be effected under atmospheric pressure conditions. The charge may be cooked as by injecting steam directly thereinto until a substantial portion of the cementitious content of the wood has been reacted upon and dissolved, but, as previously stated, the concentration of alkali in solution, the temperature, pressure, and time employed in any cook should be such as will not complete fiber liberation. After cooking has been completed, the charge may be delivered into a washer 2, wherein the wood may be separated from the spent liquor withor without washing. The spent liquor from the washer may be passed to a storage tank 3 from which a portion may be withdrawn for use together with fresh caustic soda solution in subsequent cooking of fresh wood. The rest of thespent liquor may be subjected to treatment for the recovery of its valuable inorganic content, as will hereinafter be described. The wood from the washer 2 may be delivered into a digester 4 of the type customarily employed for eflecting fiber liberation in alkaline liquors under heat and pressure. To this wood may then be added a solution of sodium sulphide in amount sufficient to cover it. inasmuch as a substantial proportion of the resinous and ligneous matter present in t e original wood has been removed, fiber li eration may be effected in a liquor of lower alkalinity than that usually employed whenthe usual onestcp cooking process is employed. lit desired, however, the solution may be of an alkali content similar to that of the usual kraft cooking liquors, in which case fiber liberation may be eli'ected in a shorter time than ordinarily. Using a sodium sulphide solution of an alkali concentration of 2 to 2.5 pounds per cubic foot calculated as N220 and cooking at a temperature of about 300 to 335 F., it is possible to produce a high grade pulp in a period of about three hoursl After fiber liberation has been effected, the digester contents may be discharged into a washer 5, wherein a separation of pulp from spent liquor may be effected. The spent liquor may be passed to a storage tank 6, from which a portion may be withdrawn for use together with fresh sodium sulphide solution in subsequent cooking of other batches of wood coming from the washer 2. The rest of the spent liquor may be mixed with the excess spent caustic soda liquor accumulating in the storage tank 3, and the mixture serving to remove a portion of the aqueous content of the mixture and thus make possible an economical recovery of its valuable inorganic content. The concentrated mixture may be fed into a furnace 8 maintained under reducing conditions, wherein the combustible content of the liquor may beburned and its valuable inorganic content sm'elted to produce a smelt consisting essentially of a mixture of sodium carbonate and sodium sulphide. The hot smelt may be delivered into a body of water maintained in a dissolving tank 9, the hot aqueous solution produced preferably being of a concentration permitting selective crystallization of sodium carbonate when cooled. Crystallization may be accelerated as by passing the hot smelt solution through a cooler 10, the resulting sodium carbonate crystals being removed from the mother liquor in which the more soluble sodium sulphide remains in solution. The sodium carbonate crystals associated with but little sodium sulphide may be dissolved in water in a tank 11, to produce a sodium carbonate solution which may then be causill ,ticized in a tank 12 to form-a caustic soda solution. If; desired, however, the sodium carbonate solution may be removed along with the desired amount of sodium sulphide,

then dissolved in water and causticized, so 935 that the first cook, which is carried short of fiber liberation, may be edected in a liquor containing much more caustic soda than sodium sulphide, and the second cook, which is carried to complete fiber liberation, may be ml effected in a liquor containing much more sodium sulphide than caustic soda. The causticized solution may be diluted in a tank 13 with water to form a fresh liquor of alkali concentration desired for use in cooking fresh Wood in the digester 1. The mother liquor, containing sodium sulphide and but a rela tively small amount of sodium carbonate, may be diluted in a tank 14 to the strength desired for use in the digester 4, its sodium carbonate content being causticized. if desired, before such use. Losses occurring in the cycle may be made up by adding sodium and sulphur constituent as in the form of sodium sulphate to the concentrated mixture of spent liquors being delivered to the furnace 8. Or, if desired, sodium losses may be made up by adding fresh caustic soda to the-solution in the tank 13, and sulphur losses may be made up by adding sulphur to the mixture of spent liquors.

Une of the important advantages of the process herein described is that it makes possible the production of strong and comparatively easily bleachable pulps. Experiments indicate that sodium sulphide'is particularly effective as a delignifying agent. For instance, if raw cellulosic material, such as spruce wood chips, is cooked under the usual temperature, pressure, and time conditions til!) aeaaees until fiber liberation is completed in two alkaline liquors of similar alkali content calculated as Na O, in one liquor all the alkali being present in the form of caustic soda and in the other liquor say about 40% of the alkali being present in the form of'sodium sulphide and the rest in the form of caustic soda, it is found that the pulp produced in the sodiumsulphide-containing liquor is stronger and has a higher cooking stain (i. e., is more easily bleachable) than the pulp produced in the all-caustic soda liquor. In other words, sodium sulphide is apparently more eifective and selective in its action upon lignin than caustic soda. With this in mind, it is thus seen that if a portion only of the lignin and resin of raw wood is removed by cooking in a liquor containing largely caustic soda, a substantially all-sodium sulphide liquor then being used to cook the wood to complete fiber liberation, the resulting pulp will be stronger and more easily bleachable than when fiber liberation is carried out to completion in an all-caustic soda liquor or in a liquor containing caustic soda in excess of sodium sulphide.

llhe example hereinbefore'given deals with the use of wood as the raw cellulosic material, but the process of the present invention may be applied in the production of pulp from other'raw cellulosic materials, such as straw, corn stalks, bagasse, and the like.

Having thus described the process of the present invention and certain of its practical applications, it should be evident to those skilled in the art that various changes and modifications might be made therein Without departing from the spirit or scope of invention as set forth in the appended claims.

I claim:

' 1. A process which comprises cooking raw cellulosic material in a solution of "caustic soda short of effecting fiber liberation, and then cooking in a solution of sodium sulphide to complete fiber liberation.

2. A process which comprises cooking raw cellulosic material in a solution of caustic soda short of efiecting fiber liberation, then cooking in a solution of sodium sulphide to complete fiber liberation, mixing the spent liquors resulting from both cooking steps, and recovering the inorganic content of the mixture.

3. A. process which comprises cooking raw cellulosic material short of efiecting fiber liberation in a mixed caustic soda and sodium sulphide solution in which the caustic soda preponderates in amount, and then cooking to complete fiber liberation in a mixed caustic soda and sodium sulphide solution in which the sodium sulphide p'reponderates in amount.

l. A cyclic process which comprises cooking raw cellulosic material in a solution of caustic soda short of effecting fiber liberation, then cooking in a solution of sodium sulphide to complete fiber liberation, mixing the spent liquors resulting from both cooking steps, smelting the inorganic content of the mixture in a reducing atmosphere to produce sodium carbonate and sodium sulphide,- separating sodiuin carbonate from the smelt,

causticizing the sodium carbonate for use in such subsequent first-mentioned cooking of wood, and using the rest of the smelt for such subsequent second-mentioned cooking.

cyclic process which comprises cook- 7 solving the smelt in water to form an aqueous smelt solution, selectively crystallizing sodium carbonate from the smelt solution, causticizing the sodium carbonate for use for such subsequent first-mentioned cooking of wood, and using the rest of the smelt solution for such subsequent second-mentioned cooking oi: wood.

I 6. A cyclic process which comprises cooking raw cellulosic material in a solution of caustic soda short of effecting fiber liberation, then cooking in a solution of caustic soda to complete fiber liberation, mixing the spent liquors resulting from both cooking steps, smelting the inorganic content of the mixture in a reducing atmosphere to produce sodium carbonate and sodium sulphide, dissolving the smelt in water to form an aqueous smelt solution, selectively crystallizing sodium carbonate from the smelt solution, causticizing the sodium carbonate for use in such subsequent first-mentioned cooking of wood, and causticizing the rest of the smelt solution for such subsequent second-mentioned cooking of Wood.

in testimony whereof l have afixed my signature.

GEQRGE A. RICHTER.

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